The present invention relates to superconductive devices employing Josephson tunneling junctions.
Josephson tunneling junctions are based upon the coupling of macroscopic quantum states in two superconductors separated by an extremely thin potential barrier. When a predetermined junction current threshold is exceeded, or a magnetic field is applied in the vicinity of a junction through which a supercurrent is flowing, it switches abruptly from a zero voltage state to a finite voltage state, 2.DELTA./e, the energy gap voltage of the junction. The magnetic flux may be supplied by one or more control elements, which are conductors overlying the junction, but separated therefrom by an insulator which is too thick to support tunneling. In addition to performing switching functions, Josephson tunnel junctions can perform memory functions by storing magnetic flux in quantized units; cf. Zappe, "A Single Flux Quantum Josephson Junction Memory Cell," Applied Physics Letters, Vol. 25, pp. 424-426 (Oct. 1, 1974), and Gueret, "Experimental Observation of the Switching Transients Resulting from Single Flux Quantum Transitions in Superconducting Josephson Devices," ibid., pp. 426-428. Memory applications rely upon the existence of many overlapping modes, called vortex modes, first described by Owen and Scalapino in Physical Review, Vol. 164, pp. 538-544 (Dec. 10, 1967).
Superconducting interferometer structures, comprising at least a pair of Josephson junctions interconnected in a superconducting loop, are known to exhibit many properties similar to those of individual junctions. In particular, switching and flux-storage functions can be performed with interferometers, in much the same manner as with individual junctions, the primary difference being that in an interferometer the applied magnetic field is chiefly coupled to the superconducting loop rather than to the junctions. For a given device size the coupling mechanism of an interferometer allows a greater sensitivity to a given magnetic field than is obtainable with an individual tunnel junction of the same total dimensions; for this reason interferometers appear very attractive from a miniaturization point of view.
As with other electronic devices made in integrated form, it is desirable to reduce the size and power dissipation of Josephson devices as much as possible. However, certain effects have imposed limits to size and power reductions. To store a single flux quantum in an interferometer, it is necessary that EQU LI.sub. o.perspectiveto. .PHI..sub.o
where L is the loop self inductance, I.sub.o is the maximum supercurrent of the total structure, and the flux quantum is .PHI..sup.o =h/2e=2.07.times.10.sup.-.sup.15 Weber. Thus, if the current level is decreased in attaining smaller size and power levels, the maintenance of the ability to store a flux quantum requires that the loop inductance be increased.
The conductors in such structures are conventionally treated as striplines whose inductance is proportional to the ratio of length to width. Miniaturization of a stripline allows the total inductance to remain constant, since length and width may be reduced proportionately. However, the capacitance, C, of a stripline is proportional to the product of length and width. Hence, capacitance decreases with miniaturization, and the characteristic impedance, proportional to the square root of the ratio of inductance to capacitance, increases. Moreover, Josephson junctions are essentially constant-voltage devices; the voltage drop 2.DELTA.e being independent of the junction size. Hence, current levels in Josephson circuits decrease naturally as the circuits are miniaturized. In addition, for a given Josephson current density, the maximum supercurrent which may be passed through a junction decreases as junction dimensions are decreased. For these reasons, the current, I.sub.o, and the product LI.sub.o decrease with smaller device sizes. A point is ultimately reached where a single flux quantum can no longer be sustained. This has previously appeared to impart fundamental limitations to the miniaturization and power reduction of superconducting, interferometer devices.
Apart from the factors which show the need for more inductance as devices grow smaller, it should be appreciated that the phenomenon of "kinetic inductance" has been seen in connection with experiments on thin films. It was discovered that inductance could be enhanced by making the thickness of the film less than the superconducting penetration depth (.lambda.) of the film at superconducting temperatures. This phenomenon was first reported in the "Proceedings of the Symposium on the Physics of Superconducting Devices," April 1967, U. of Va., Charlottesville, Va., in a paper by W. A. Little entitled "Device Application of Super-Inductors."
Typical prior art interferometer structures are discussed in Physical Review, Vol. 140, No. 5A, Nov. 29, 1965, p.A1628 in an article entitled "Macroscopic Quantum Interference in Superconductors" by R. C. Jaklevic et al, and in Physical Review B, Vol. 6, No. 3, Aug. 1, 1972, p. 855 in another article entitled "Quantum Interference Properties of Double Josephson Junctions" by T. A. Fulton et al. The structures shown do not affirmatively incorporate the use of kinetic inductance and, to the extent that a hindsight view of these structures might suggest that such teaching was accidentally incorporated or utilized as a correction factor, the enhancement ratios obtained are always less than one.